Poly(ethylene terephthalate) fibers accounts for the main part of the polyester applied by the textile industry. The fibers are produced by e.g. poly-condensation of terephthalic acid and ethylene glycol, and drawing of fibers from a melt. During these processes, at high temperatures, cyclic oligomers, in particular cyclic tri(ethylene terephthalate), are formed in and on the fibers.
Cyclic oligomers tend to give fabrics with a content of poly(ethylene terephthalate) fibers a grayish appearance. This is due to deposits of cyclic oligomers on the surface of the fabric, which is particularly outspoken after high temperature wet processes like HT (high temperature) dyeing. The cyclic oligomers are difficult to remove and may even be resistant to an alkaline post treatment [cf. G. Valk et al.; Melliand Textilberichte 1970 5 504-508]. Therefore, to be effective, the alkaline treatment has to be severe, which results in a significant loss of fiber material. Also, organic extraction of the cyclic oligomers is a technical possibility, but not industrially feasible.